Hard disk

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Top and bottom views of a Western Digital WD400 3.5" hard disk

A hard disk drive (HDD) , hard disk (sometimes 'disk' is also spelled disc) or hard drive, is something used by computers to store information. Hard disks use magnets to store information. The capacity of a hard drive is usually measured in gigabytes (GB). A gigabyte is one thousand megabytes and a megabyte is one million bytes, which means that a gigabyte is one billion bytes. Some hard drives are so large that their capacity is measured in terabytes, (TB) where one terabyte is a thousand gigabytes (1 TB = 1000 GB).

[change] Different interfaces

Serial ATA cables are a common way to connect hard disks

Hard disks use different interfaces and ports. An interface usually has two parts. First there is the specification of how the signals are transmitted. This includes what cables to use, what length the cables can be and so on. The second part of the interface definition is the language to use. This speaks about what the signals mean.
As there are different kinds of hard disks, there are also different kinds of interfaces. In 2009, it was common to attach a hard disk using a Serial ATA connection. The connection that came before that was called "IDE" and is called Parallel ATA today. In large data centers, Fibre Channel is often used.
For servers, the SCSI (pronounced 'scuzzy') interface is very popular. There are several types and versions of SCSI interface, like parallel and Serial Attached SCSI, each stepping-up in terms of speed and price. Within servers, several SCSI drives are often used in conjunction with each other, in order to safeguard against data loss or corruption (this is known as RAID - and there are many different configurations to choose from).